A kilowatt is a rate of power: a solar system's size is quoted in kilowatts of direct-current panel capacity, such as a 7 kW system. A kilowatt-hour is an amount of energy: one kilowatt sustained for one hour. Your utility bill charges for kilowatt-hours, and a solar system's value depends on how many kilowatt-hours it produces in a year. The link between the two is the production per kilowatt for your location, which depends on sunlight, roof direction, tilt and shading. A sunny desert city produces noticeably more energy per kilowatt than a cloudy northern one, so the same system size saves different amounts in different places. Price quotes are compared fairly in dollars per watt of capacity.
Related terms and pages
- Price per watt: Total installed price divided by system size in watts: the fairest way to compare quotes.
- PVWatts: A free model from the national laboratory formerly called NREL that estimates solar output for any location.
- Capacity factor: Actual output over a year divided by what the system would make running at full power all year.
- Solar calculator